1. Field of the Invention
The present invention relates to digital broadcast receiving apparatuses and, more particularly, to a digital broadcast receiving apparatus configured to receive both satellite digital broadcasting and terrestrial digital broadcasting.
2. Description of the Related Art
Recently, the digitization of broadcasting has been speeded up, with the satellite digital broadcasting implemented from 2000 by use of the SHF band. Also, from December 2003, the terrestrial digital television broadcasting started by use of the UHF band. In addition, the experimental broadcasting of the terrestrial digital audio broadcasting has started by use of the VHF band.
In the service form of the satellite digital broadcasting, each of the programs produced by broadcasting companies A, B, and so on is transmitted to an uplink center in the form of TS (Transport Stream) of MPEG-2 (Motion Picture Experts Group-2) to be time-division multiplexed, the multiplexed program being transmitted to a transmitter station. In the transmitter station, a carrier having a predetermined frequency is modulated by the time-division multiplexed transport stream into transmission signal, which is transmitted to a BS (Broadcasting Satellite) via a transmission antenna.
In the broadcasting satellite, the frequency of the transmission signal received from the transmitter station is converted into a frequency of the broadcasting band of the satellite digital broadcasting and then resultant signal is amplified to be transmitted to the ground. The transmission signal transmitted from the transmitter station is received at a reception antenna to be supplied to a satellite digital broadcast receiving apparatus. It should be noted that, at the present time (year 2004), four channels, namely, four carrier frequencies, are used for the satellite digital broadcasting in Japan.
In a relation between the channel and frequency for use in the satellite digital broadcasting, channel 1 (BS1), channel 3, channel 13, and channel 15 in the 12-GHz band allocated to the Japanese satellite broadcasting are used for digital broadcasting.
In the satellite digital broadcasting, a broadcast signal is retransmitted by use of two or more transponders corresponding to channels arranged in a broadcasting satellite. Broadcast signals for one channel are transmitted from the satellite by use of one transponder. If there are eight satellite broadcasting channels, eight transponders are used for the satellite digital broadcasting. At the present, four of the eight channels are used for the satellite digital broadcasting and the remaining four channels are used for the satellite analog broadcasting.
The broadcast signals for one channel of the satellite digital broadcasting include two or more broadcasting services. For the types of broadcasting services included in channel 1 (BS1), the signals to be transmitted on channel 1 (BS1) are divided into two or more transport streams, each transport stream being allocated to each broadcasting company for broadcasting. Channel 1 (BS1) contains the television broadcasting, the audio broadcasting, and the data broadcasting at the same time.
The channels of the satellite digital broadcasting have frequencies different to each other, so that, in each satellite digital broadcasting apparatus, an incorporated tuner is controlled so as for the signal having the frequency of a desired channel to be received, thereby receiving the broadcast signal of the channel 1 (BS1).
As described above, the transport streams of the broadcast signals of channel 1 (BS1) includes the information of the television broadcast, the audio broadcasting, and the data broadcasting, so that all transport streams of the broadcast signals of channel 1 (BS1) are received and only the transport streams of a predetermined portion are extracted for decoding.
Consequently, any of the television broadcasting, the audio broadcasting, and the data broadcasting can be received by one type of tuner. Therefore, in the satellite digital broadcast receiving apparatus, the audio broadcasting and the data broadcasting can be received by use of a circuit configured to receive the television broadcasting.
As with this satellite digital broadcast receiving apparatus, terrestrial digital broadcast receiving apparatuses for receiving the terrestrial digital broadcasting are configured to receive any of the television broadcasting, the audio broadcasting, and data broadcasting with tuners of one type.
Referring to FIG. 16, there is shown a schematic diagram illustrating a tuner circuit and a demodulating circuit in a related-art digital broadcast receiving apparatus. The related-art digital broadcast receiving apparatus has three circuits; a terrestrial digital broadcast tuner circuit 151, a satellite digital broadcast tuner circuit 152, and a demodulating circuit 153 configured to demodulate an IF (Intermediate Frequency) signals 166 and 169 of the reception signals of these two schemes.
The IF signals 166 and 169 of the reception signals of these two schemes are analog signals, so that these signals are converted by an A/D converting circuits 160 and 162, respectively, into digital signals in the preprocessing of the demodulating circuit 153. The demodulating circuit 153 demodulates there digital signals. The demodulating circuits 153 has a terrestrial digital broadcast tuner circuit control block 154 and a satellite digital broadcast tuner circuit control block 155 configured to execute control in the circuit.
The terrestrial digital broadcast tuner circuit control block 154 and the satellite digital broadcast tuner circuit control block 155 have an AGC (Automatic Gain Control) 161 and an AGC 163, respectively. AGC signals of the AGC 161 and AGC 163 are fed back to the terrestrial digital broadcast tuner circuit 151 and the satellite digital broadcast tuner circuit 152 via a terminal 156 and a terminal 158, respectively. Control is executed such that the AGC signals are passed to an intermediate frequency amplifying circuit in the tuner circuit 151 and an intermediate frequency amplifying circuit in the tuner circuit 152 to be automatically amplified to an optimum level in these amplifying circuits. The control for optimizing the IF signal amplitude consequently widens dynamic range DR of the A/D converting circuits 160 and 162.
Serial communications signals 165 and 167, such as various setting data, are bidirectionally communicable between the terrestrial digital broadcast tuner circuit 151 and the satellite digital broadcast tuner circuit 152 and the terrestrial digital broadcast tuner circuit control block 154 and the satellite digital broadcast tuner control block 155 via the terminals 157 and 158, respectively.
A digital demodulating circuit configured to replace terminals of a digital modulating circuit has also been proposed (refer to Japanese Patent Laid-Open No. 2003-125302, hereinafter referred to as Patent Document 1). Namely, if the reception of two broadcast programs is desired and these two broadcast programs are on different frequency channels, then baseband signals (I signal and Q signal) outputted from two lines of tuner blocks are supplied to two lines of demodulating blocks, respectively.
If two broadcast programs of which reception is desired are on one frequency channel, then a signal switching circuit operates such that the baseband signals (I signal, Q signal) are switched so as to supply the baseband signals (I signal, Q signal) outputted from one line of tuner block to two lines of demodulating blocks.